Traffic signals, also known as traffic lights, traffic lamps, signal lights, stop lights, or traffic control signals, are signaling devices positioned at road intersections to control competing flows of traffic. Traffic signals typically alternate the movement of the flows of vehicles by displaying lights of standard colors (e.g., red, yellow, and green). Traffic signals may use timers to implement fixed intersection cycling times. For example, a first traffic flow travelling in a particular direction may be permitted to pass through an intersection during a first time period and may be stopped during a second time period. The cycling times may vary based on the time of day and day of the week, or for other special circumstances such as a major event.
Traffic signals may be dynamically controlled based on detected traffic patterns. The traffic patterns may be detected, for example, using pressure sensors in a road or other vehicle sensors. Traffic signals may give more time to a traffic flow associated with heavy traffic, and the traffic signals may shorten or even skip another traffic flow that has little or no traffic waiting.
Traffic signals may give priority to special traffic, particularly emergency vehicles such as fire trucks, ambulances, and police cars. For example, a traffic signal may include a sensor to receive a radio-frequency signal and/or sounds transmitted by an emergency vehicle and may change based on detecting the emergency vehicle.